1. Field of the Invention
This invention relates generally to an astable multivibrator circuit and more particularly to such a circuit readily adopted to be used in the so-called "chopping" type DC power supply circuit.
2. Description of the Prior Art
An astable multivibrator circuit is well known in the art as a square wave generator or oscillator. This circuit finds use in many different kinds of electric devises and systems. A conventional astable multivibrator circuit usually includees a pair of active elements such as transistors, electron tubes and the like which are switched on and off alternately and reciprocally. Output signals of square or rectangular waveform are obtained from either or both of the active elements.
Such an astable multivibrator circuit is often utilized as a voltage level controlling circuit in the so-called "chopping" type DC power supply circuit. In this type of power supply circuit, the astable multivibrator circuit is controlled to produce an output pulse signal of variable pulse width for driving a chopping switcher, so as to stabilize an output DC voltage from the power supply circuit.
However, when the conventional astable multivibrator circuit is controlled to produce an output signal of variable pulse width, the frequency of the output signal usually is subjected to a concurrent change. This change in the frequency of the output signal is usually undesired, especially when the astable multivibrator circuit is utilized in the "chopping" type DC power supply circuit. In particular, frequency changes cause an increase in the power loss of switching devices, transformers, etc. included in the power supply circuit, with the strong possibility that the switching devices are apt to overheat resulting in damage and even destruction thereof
If this type of conventional power supply circuit is used in a television receiver, the change in the frequency of the output signal form the astable multivibrator can deleteriously affect the quality of the video picture reproduced on the screen of the cathode ray tube of the television receiver.